1. Field of the Invention
This relates generally to the storage of data on magnetic mass storage media and, more particularly, to a digital group coded recording (GCR) data recovery system for receiving GCR data stored on magnetic tape.
2. Description of the Prior Art
Group coded recording (GCR) is generally employed in the storage of digital data on magnetic mass storage media (e.g. magnetic tape) and involves the encoding of data so that a single bit stream will contain no more than two successive 0's throughout the data record. The coded data is then written on the magnetic medium at some predetermined consistent spacing such that a "1" is represented by a magnetic flux reversal and a "0" is represented by the absence of a magnetic flux reversal.
U.S. Pat. No. 3,639,900 shows an error detection and correction system for use in GCR systems. It is cited as background material to put the invention of the present application in perspective with respect to the larger GCR system. The invention of the pending application could be used in the system of the cited reference between elements 13 and 14 in FIG. 1 therein. The invention disclosed here would improve data recovery from relatively "dirty" signal from the magnetic media by comparing the incoming signal with the average output data rate from the invention so as to better predict when an incoming signal represents a valid flux reversal on the magnetic media. The invention utilizes the data from this comparison to predict the actual data recorded on the magnetic media and to signal the proper time and polarity of an output data bit generated by the output means described herein.
It has heretofore been known in the industry to recover GCR data through the use of analog equipment. The most common method employes a variable frequency oscillator running at a frequency which is maintained at a multiple of the data rate which is being read. In addition to the variable frequency oscillator, a phase sensing device is employed to control the sampling position of the data. This "analog" approach suffers from several disadvantages. First, the analog approach requires unique test equipment and highly trained personnel to effect proper operation, trouble shooting and repairs. Second, the analog equipment experiences some degree of frequency drift resulting in variations in performance and requiring periodic recalibration. Third, the equipment requires the inclusion of components which permit tuning and adjusting to effect compatibility with different data rates.
It is also known to employ a digital data recovery method in the recover of phase encoded data stored on magnetic tape. However, this approach has been used only with relatively low data frequencies i.e. 3,200 bits per inch when no more than one successive "0" is contained in valid data and wherein no data variations are encountered. This approach is not applicable to GCR wherein substantial data variations are encountered specifically in relation to peak shift and rapid excursions of the data signal which are not representative of valid data and wherein the data frequencies are substantially higher (i.e. 9,200 bits per inch).